1. Field of the Invention
The present invention relates to an OFDM communication apparatus and propagation path estimation method in a digital radio communication system.
2. Description of the Related Art
A leading cause of deterioration of a transmission characteristic in a current ground wave transmission path is multi-path interference. An OFDM (Orthogonal Frequency Division Multiplexing) transmission system, which is resistant to this multi-path interference, is receiving attention in recent years. This OFDM is a system that multiplexes a multiple (several tens to several hundreds) of mutually orthogonal digital modulated signals in a certain signal segment.
A conventional OFDM communication apparatus calculates a frequency response estimated value of the propagation path by performing time-frequency conversion on a reception signal through an FFT circuit and performing a complex multiplication on a pilot symbol and known signal contained in the reception signal. Then, by carrying out a complex multiplication on the frequency response estimated value and information OFDM symbol, the conventional OFDM communication apparatus compensates propagation path distortion. This reception signal with propagation path distortion compensated is demodulated and subjected to error correction by an error detection circuit and in this way an information bit string, which is reception data, is obtained.
When long information is transmitted, the conventional OFDM communication apparatus above inserts, as shown in FIG. 1, propagation path response estimation pilot symbols (hatched areas) into information OFDM symbols at certain intervals to follow up variations of momentarily changing propagation path response. That is, as shown in FIG. 2, information OFDM symbols 1 to n are compensated using a propagation path estimated value obtained by pilot symbol A and information OFDM symbols n+1 to 2n are compensated using a propagation path estimated value obtained by pilot symbol B.
However, when such long information is transmitted, following up time variations in the propagation path makes it necessary to frequently insert known signals such as pilot symbols, causing a problem of deteriorating the transmission efficiency.
It is an object of the present invention to provide an OFDM communication apparatus and propagation path estimation method capable of adaptively following up time variations of a transmission path and improving a reception characteristic without reducing the transmission efficiency even if there are considerable time variations in propagation path response.
A subject of the present invention is to adaptively follow up time variations of a transmission path and achieve an excellent reception characteristic by adaptively estimating propagation path response using an error-corrected signal, that is, using a judged value of the received information signal as a known signal without frequently inserting pilot symbols for propagation path estimation and without reducing the transmission efficiency even when long information is transmitted.